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Moore’s Law in Homeland Defense: An Integrated Sensor Platform Based on Silicon Microcantilevers Lal A. Pinnaduwage, Member, IEEE, Hai-Feng Ji, and Thomas Thundat
Abstract—An urgent need exists for the development of inexpensive, highly selective, and extremely sensitive sensors to help combat terrorism. If such sensors can be made miniature, they could be deployed in virtually any situation. Terrorists have a wide variety of potential agents and delivery means to choose from for chemical, biological, radiological, or explosive attacks. Detecting terrorist weapons has become a complex and expensive endeavor, because a multitude of sensor platforms is currently needed to detect the various types of threats. The ability to mass produce and cost effectively deploy a single type of sensor that can detect a wide range of threats is essential in winning the war on terrorism. Silicon-based microelectromechanical sensors (MEMS) represent an ideal sensor platform for combating terrorism because these miniature sensors are inexpensive and can be deployed almost anywhere. Recently, the high sensitivity of MEMS-based microcantilever sensors has been demonstrated in the detection of a variety of threats. Therefore, the critical requirements for a single, miniature sensor platform have been met and the realization of an integrated, widely deployable MEMS sensor could be near. Index Terms—Chemical, biological, radiological, or explosive (CBRE) detection, homeland defense, microcantilever, microelectromechanical sensors (MEMS) sensor, terrorism.
I. INTRODUCTION
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ERRORISTS have a huge economic advantage over law enforcement because it is, many times, more expensive to detect terrorist threats than it is to deploy terrorist threats. For example, a crude explosive device can destroy an airplane in flight. On the other hand, current explosive detection technologies deployed at airports are expensive and require constant operator attention. A chemical or biological attack, which can also be carried out with nominal cost and effort, might even go unnoticed until injured people start turning up at hospitals. On the nuclear side, a “dirty bomb,” which uses radioactive material that will be spread using a conventional bomb, is another likely threat. Even though sensitive detection of individual threats may be currently possible, such techniques/sensor systems are bulky, expensive, and require time-consuming procedures. Also, Manuscript received February 3, 2004; revised September 2, 2004. This work was supported in part by the Bureau of Alcohol, Tobacco, and Firearms (ATF), in part by the National Safe Skies Alliance, in part by the Department of Homeland Security, in part by the Department of Energy’s NA-22 program, in part by the Environmental Management Science Program, and in part by the Office of Biological and Environmental Research Program. The associate editor coordinating the review of this paper and approving it for publication was Dr. Timothy Swager. L. A. Pinnaduwage and T. Thundat are with the Life Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831-6122 USA, and also with the Department of Physics, University of Tennessee, Knoxville, TN 37996-1200 USA (e-mail:
[email protected]). H.-F. Ji is with the Institute for Micromanufacturing, Louisiana Tech University, Ruston, LA 71272 USA. Digital Object Identifier 10.1109/JSEN.2005.845517
detection of multiple threats requires the use of a variety of specialized instruments based on different technologies/sensor platforms. Therefore, a paradigm change in sensor technology is required for combating the war on terrorism. Ideally, homeland defense requires a sensor system with the following features: 1) a single-sensor platform that can detect multiple threats simultaneously and rapidly; 2) an inexpensive, miniature, and robust sensor system that can be deployed almost anywhere; and (3) built-in telemetry for data transmission and networking. None of the available technologies satisfy these conditions. However, the emerging sensor technology based on MEMS has the promise of satisfying these conditions. Such miniature sensors could be deployed anywhere—airports, seaports, public buildings, strategic locations in waterways—providing omnipresent protection. The technical advances in a variety of fields, including computing, interphase chemistry, and telemetry, have matured enough to be incorporated into MEMS sensor technology. Thus, the MEMS sensor platform is poised for such a revolution in sensor technology. For example, present sensor technology can be compared with the status of computer use in the 1960s when any serious computing was restricted to giant computers installed in a handful of institutions. But the “silicon revolution,” expressed succinctly in Moore’s Law [1], has enabled widespread computer use. Currently, expensive and bulky detection systems are sparsely deployed at strategic locations, such as airports. An analogous revolution in sensor technology may be possible with a sensor platform based on MEMS that will allow deployment of intelligent, miniature sensors by the millions. Besides the war against terrorism, such a sensor platform would be useful in medical diagnostics, law enforcement, landmine detection, environmental monitoring, and many other applications. Therefore, the primary issue can be stated as follows: A need exists for rapid detection of trace quantities of a wide variety of threat agents present in complex mixtures using miniature, inexpensive sensors. Here we discuss the current status of research on achieving this goal with sensor arrays and point out that microcantilever MEMS sensors provide a suitable platform. It must be noted that this paper is not intended to be a comprehensive review paper. We will briefly review the current status of sensor arrays and refer to selected papers on microcantilver and other sensor technologies. Our intention is to point out the possibility of achieving a miniature sensor platform for homeland defense based on microcantilever sensors. II. SIMULTANEOUS AND RAPID DETECTION OF MULTIPLE ANALYTES For just over two decades, research has been conducted on development of an “electronic nose” based on sensor arrays
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[2]–[6]. In 1982, Persaud and Dodd [7] published the first paper on a modern electronic nose that attempts to mimic the olfactory system: They pointed out [7] that the mammalian olfactory system is based on broadly tuned receptor cells, and that the discrimination properties of the olfactory system are a property of the system as a whole. The olfactory receptors are not highly selective toward specific odorants; each receptor responds to multiple odorants, and many receptors respond to any given odorant [8]. Pattern recognition methods are thought to be a dominant mode of olfactory signal processing. The electronic nose technologies are based on the same concept of using broadly tuned multiple sensors. An advantage of this approach is the ability to detect a variety of analytes simultaneously. Several possible sensor array platforms have been studied up to now [2], [9] as candidates for an electronic nose, including those based on metal oxide, MOSFET, conductive polymer, fiber-optic, electrochemical, and acoustic wave sensors. These sensor technologies are described in detail by Gardner and Bartlett [6]. A wide variety of statistical and neural network techniques have been employed to process the data originating from the sensor arrays [6]. It is not surprising that one of the first applications of electronic noses based on sensor arrays has been for the evaluation of odors and for industrial process control [6]. Sensor arrays have been shown to be successful in these applications in which the primary interest is in qualitative analyzes that rely on changes in the sensor-array response patterns. Commercial instruments based on sensor arrays are available for these applications [6]. The commonly used sensor technologies in these applications are metal-oxide, conducting polymer, and acoustic [surface acoustic wave (SAW) and QCM] sensors [6]. Metal oxide sensors are bulky, but they have fairly good sensitivities of sub-parts-per-million (ppm) levels; conducting polymer sensors are small with low-power consumption, but the sensitivities are generally an order of magnitude lower compared with metal oxide sensors; SAWs sensors derive their vapor-detecting capability from sorbent coatings and can detect the mass of the vapor adsorbed, which in some cases can be distorted by the changes of the viscoelastic properties of the coating material [8]. However, the detection of trace amounts of vapors in mixtures has not yet been successfully achieved with sensor arrays [6], [10]. Almost all sensor array studies conducted up to now have used sensors with detection limits at or near 1 ppm level and a maximum of about 12 sensors in an array [6], [10]. Better detection sensitivities and higher numbers of sensors per array may be needed to achieve trace detection in complex mixtures. A recent study [11] has concluded that increasing the number of sensors in an array did not improve performance significantly for mixture analysis; a maximum of six sensors were used in the array, and the detection limits of the SAW sensors used in the array for the component vapors were at low ppm levels [11]. Moreover, it has been noted that there are over 1000 olfactory genes in humans and over 100 million olfactory cells in a canine’s nose [2]. It is likely that MEMS sensors may provide both high sensitivity and the ability to use a much higher number of individual sensors in an array, thus enabling the detection of trace vapors in complex mixtures. The high sensitivity for MEMS sensors
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originates in the inherently large surface-to-volume ratio of the microscopic objects. Thus, a MEMS sensor based on surface interactions for signal transduction can be expected to provide an enormous amplification in sensitivity. The rapid development of the integrated circuit (IC) technology during the past decade has initiated the fabrication of chemical sensors on silicon or complementary metal oxide semiconductor (CMOS) [12]–[14]. The largely two-dimensional integrated circuit and chemical sensor structures processed by combining lithographic, thin-film, etching, diffusive, and oxidative steps have been recently extended into the third dimension using micromachining or MEMS technologies—a combination of special etchants, etch stops, and sacrificial layers [12]. Therefore, MEMS technology provides an excellent means to meet other key criteria of chemical sensors, such as miniaturization of the devices, low-power consumption, and batch fabrication at low cost as well. Currently, two CMOS-technology-based MEMS sensors are being studied, flexural plate wave (FPW) sensors and microcantilever sensors. The FPW sensor is similar in many ways to the more common SAW and QCM sensors, and it can monitor the mass absorbed on a coating deposited on the sensor [15], [16]. It has shown detection sensitivities of high parts-per-trillion (ppt) levels at the highest measured sensitivity level [15]. On the other hand, the microcantilever sensors have additional detection modes [17], including the highly sensitive bending mode that makes use of the high surface-to-volume ratio of the MEMS sensor element and often displays sub-ppt detection sensitivities as discussed in Section III.
III. MICROCANTILEVER SENSORS In 1994, researchers observed that the microcantilevers used in atomic force microscopy (AFM) were sensitive to external physical and chemical influences. Thundat et al. [18]–[21] pointed out the possible use of bending and frequency shift in microcantilevers for chemical sensing, and Gimzewski et al. [22]–[24] pointed out applications in thermal calorimetry. Since then, researchers all over the world have been reporting the use of microcantilevers for detecting various physical, chemical, biological, and radiological influences [17], [25]. Microcantilevers are miniature diving boards that are micromachined from silicon or other materials. The length of these cantilevers is often in the range of 100–200 m, whereas the thickness ranges from 0.3 to 1 m. (It is interesting to note that in the human olfactory system, the site of interaction of the cell with odorous molecules occurs at hairlike cilia, which are up to 200- m long and provide an increased surface area for odor sensing [6].) The key to the high sensitivity of the microcantilevers is the enormous surface-to-volume ratio, which leads to amplified surface stress, as discussed below. Fig. 1 shows a set of microcantilevers together with a human hair for comparison. Microcantilevers have two main signal transduction methods, bending and mass-loading. In the mass-loading mode, microcantilevers behave just like other gravimetric sensors such as QCM, SAW, and FPW transducers: Their resonance frequencies decrease due to the adsorbed mass. In the “gravimetric mode,”
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Fig. 1. Scanning electron microscope image of a set of cantilevers of different sizes and shapes. For comparison, a human hair is also shown.
the microcantilever detection sensitivity seems to be comparable with that of the other gravimetric sensors [26]. The other signal transduction method, that is, the bending response, is unique to the microcantilever; for example, if a differential surface stress is achieved by preferentially adsorbing target molecules to one of its broad surfaces (by using a chemical coating on that surface), the microcantilever will bend. Since a differential surface stress is required for bending, only one broad surface should be coated for bending-mode operation. Therefore, the mass-loading information is used only as a bonus. In the bending mode, microcantilever detection sensitivity is at least an order of magnitude higher than other miniature sensors such as SAW and QCM that are also being investigated as chemical sensors. Even though it is difficult to accurately compare the detection sensitivities for different sensors, the low- or below-ppt detection sensitivities routinely achieved with microcantilever sensors [27]–[32] have not been matched by any other sensor. The closest comparison would be the detection of dinitrotoluene (DNT) using SAW [33] and microcantilever [30] sensors, where both sensors used the same polymer coating SXFA-[poly(1-(4-hydroxy-4-trifluoromethyl-5,5,5-trifluoro)pent-1-enyl)methylsiloxane]. An estimated 400-ppt detection sensitivity is achieved for 200-s exposure of DNT to the SAW sensor ([33, Fig. 10]), whereas 300-ppt detection sensitivity was achieved with the microcantilever in a few seconds [30]. Since the microcantilever bending signal originates in surface stress, diffusion of large amounts of vapor to a thick coating is not necessary. Thus, even though one-monolayer-thick self-assembled monolayer (SAM) coatings may not be appropriate for gravimetric sensors [34], they are ideally suited for microcantilever sensors. Because diffusion of the analyte vapor to the bulk of the coating is avoided, the response and relaxation of a microcantilever coated with a SAM can be fast. Fig. 2 shows the response of a microcantilever coated with a SAM of 4-mercaptobenzoic acid to a vapor stream of the plastic explosive pentaerythritol tetranitrate (PETN) at a concentration of 1.4 ppb [29]. The rapid and sensitive response of the bending signal as well as the superiority of the bending signal compared with the mass (frequency) measurement is clear.
Fig. 2. Response of a 4-mercaptobenzoic acid (4-MBA)-coated silicon cantilever to the periodic turning on (10 s) and off (60 s) of a PETN stream of 1.4-ppb concentration in ambient air. The solid curve depicts the bending response, and the dots connected by dashed lines depict the resonance frequency of the cantilever [29].
It must be emphasized that the bending of the microcantilever is not caused by the weight of the deposited material. A 40-ng microcantilever bends about 1 nm due to its own weight, which is just above the noise level for a cantilever-bending signal. Therefore, the microcantilever bending caused by the weight of the deposited material of picogram levels is insignificant. On the other hand, for micron-size objects like microcantilevers, the surface-to-volume ratio is large, and the surface effects are enormously magnified. Thus, adsorption-induced surface forces can be extremely large. The adsorption-induced force can be attributed to the change in surface free energy due to adsorption. Free energy density (millijoule/square meter) is the same as surface stress (newton/meter). This surface stress is analogous to surface tension in a liquid. Incidentally, surface stress has the units of a spring constant of a cantilever. Therefore, if the surface free-energy density change is comparable with the spring
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constant of a cantilever, the cantilever will bend. When probe molecules bind to their targets, steric hindrance and electrostatic repulsions cause the bound complexes to move apart. Because they are tethered at one end and because the surface area is finite, they exert a force on the surface. Another advantage of the microcantilever sensor is that it works with ease in air and in liquid. Both resonance frequency and bending modes can be used in liquid. Because of the small mass of microcantilevers, they execute thermal motion (Brownian motion) in air and liquid. Therefore, no external excitation technique is needed for exciting cantilevers into resonance; the degraded quality factor in liquid can be improved by a feedback mechanism [35], [36]. However, as in the vapor phase, the microcantilever bending signal is mainly used for sensing in liquid (see, for example, [37]–[39]). Despite its high sensitivity, the cantilever platform offers no intrinsic chemical selectivity just like other chemical sensors such as SAW and QCM. One surface of the silicon microcantilever can be functionalized so that a given molecular species will be preferentially bound to that surface when it is exposed to a vapor stream. Therefore, detection sensitivity is vastly enhanced by applying an appropriate coating on one cantilever surface. Such a coating can, in principle, provide selectivity as well. Basic requirements for a sensor system are sensitivity, selectivity, and reversibility. Although the sensitivity and selectivity are critical, reversibility may not be necessary under some conditions. The lack of selectivity will lead to frequent false positives (false alarms), which is as bad as false negatives due to lack of sensitivity. However, the degree of selectivity is inherently connected to the reversibility of detection. Selectivity and reversibility are often competing characteristics of chemical sensors. The type of interaction occurring between analyte molecules and the cantilever coating determines the adsorption and desorption characteristics. Low-energy, reversible interactions such as physisorption generally lack an acceptable degree of selectivity: The energies involved range kJ mol ) to from van der Vaals interactions (energy kJ mol ). Furthermore, acid-base interactions (energy the weak interaction may lead to insufficient sorption, which makes sensor response weak. At the other end of the spectrum, highly selective interactions are normally covalent in nature (chemisorption) and are not reversible (binding energies are kJ mol ) under normal conditions. Two “intermediate-range” interactions can provide limited selectivity while being reversible. One is hydrogen bonding, and the other is coordination chemistry. A hydrogen atom covalently bond to an electronegative atom will have the electron cloud shifted away from it, and thus it can form a hydrogen bond with another electronegative atom. For example, the O atoms in the characteristic nitro groups of explosives can participate in hydrogen bonding. A coordination compound consists of a central metal atom surrounded by neutral or charged, often organic, ligands. In the ligand, one or more donor atoms interact with the metal ion. The selectivity now can be influenced by the choice of the metal ions as well as by the choice of the ligand, both from an electronic or steric point of view [40]. In most applications, it is desirable to have the ability to regenerate the sensor, and thus the use of “intermediate range”
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interactions will be necessary, which in turn broadens the target range. Therefore, a single microcantilever coating may not provide sufficient selectivity if reversible sensor operation is required (one exception to this is the detection of hexavalent chromium in a complex matrix using a single cantilever [38]). However, in general, it will be necessary to use an array of microcantilevers with multiple coatings to obtain sufficient selectivity, especially if the sensor is required to monitor multiple threats. Pattern recognition schemes need to be employed to extract the composition of the target vapor stream. Much work on coating materials has been done over the years, especially in the development of SAW sensors [34], [41], [42]. In this context, various polymer coatings have been investigated [34], [41]. These polymer coatings have been optimized primarily for the use in SAW and QCM sensors in which mass loading is the key sensing parameter. This consideration has led to the development of mainly polysiloxane films that allow rapid diffusion of the analyte into the bulk of the film to provide optimum mass loading. SAM coatings have recently gained attention because of their simplicity and robustness [43], [44]. Even though SAM coatings may not provide high mass sensitivity due to the lack of “volume absorption” for SAW, QCM, and similar sensors [34], they are ideal for microcantilever sensors working in the bending mode for the following reasons: 1) the coating can be applied by a simple procedure such as soaking or touch coating [45], 2) the covalent binding of the SAM to the sensor surface provides a stable coating with direct transmission of the stress due to analyte binding to the microcantilever, and 3) because diffusion of analyte molecules to a thick coating is avoided, sensor response and relaxation is fast. Some examples of the usage of both polymer and SAM coatings with microcantilever sensors will be presented below. Over the past ten years, many breakthroughs have taken place in the area of microcantilever sensors. Advances in micromachining made it possible to develop arrays of microcantilever beams with required sensitivity. Many chemically selective coatings for chemical speciation have also been developed as discussed earlier. Receptor-ligand, antibody-antigen, or enzyme-substrate reactions have been studied for biological detection [46], [47]. Advances have also been made in many other crucial areas, such as immobilization of selective agents on cantilever surfaces and application of selective layers on cantilever arrays. Aided by such tools, physical, chemical, and biological detection has been demonstrated using microcantilever sensors. The following sections of this paper discuss representative microcantilever-based detection results that pertain to homeland security. IV. SELECTED EXAMPLES OF MICROCANTILEVER STUDIES RELEVANT TO TERRORIST THREAT DETECTION A. Explosives Most explosives have low-vapor pressures at ambient temperatures. Table I shows vapor pressures of trinitrotoluene (TNT); 2,4 dinitrotoluene (DNT); pentaerythritol tetranitrate (PETN); and hexahydro-1,3,5-triazine (RDX) at a range of temperatures. TNT is one of the more commonly used explosives. Even though DNT is not an explosive, detection of explosives can be based
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TABLE I VAPOR PRESSURES (NORMALIZED TO ATMOSPHERIC PRESSURE) OF SOME COMMON EXPLOSIVES (PARTS-PER-BILLION)
on the detection of DNT because of the following reasons. 1) Even though the main ingredient of explosives is TNT, a major impurity in production grade TNT is DNT. 2) DNT is a synthetic byproduct of TNT. 3) The saturation concentrations of DNT in air are approximately 25 times larger than TNT (see Table I). High explosives, such as PETN and RDX, are the most serious threats in aircraft sabotage because they can be easily molded for concealment (the infamous “shoe bomber” had PETN hidden in his shoes), are very stable in the absence of a detonator, and can in small amounts destroy a large airplane in flight. They are, in fact, the explosives most commonly used for this purpose. We are pursuing two parallel schemes for the detection of explosive vapor. One is based on the commonly used “passive detection,” where a microcantilever coated with a suitable substance adsorbs the explosive molecules preferentially to one surface that leads to the bending of the cantilever. Using this approach, we have reported the detection of plastic explosive vapors using a 4-mercaptobenzoic acid SAM coating [29] and the detection of dinitrotoluene using SXFA-[poly(1-(4-hydroxy-4-trifluoromethyl-5,5,5-trifluoro)pent-1-enyl)methylsiloxane]-polymer-coated microcantilevers [30]. Datskos et al. [51] reported the detection of TNT and DNT with microcantilevers with nanoporous coatings. The second approach employs a novel “active detection” method in which explosive vapors are deposited on uncoated piezoresistive microcantiver surfaces, and a pulsed voltage is applied to the microcantilever to heat it to high temperatures where the deposited explosive material undergoes deflagration [52], [53]. Because the deflagration event only occurs for explosive material [54], this method yields unambiguous detection of explosives. A vapor generator developed at the Idaho National Engineering and Environmental Laboratory (INEEL) was used to generate the PETN, RDX, and TNT vapor streams. Flowing ambient air through a reservoir containing a small amount of the explosive material generated the vapor stream. Fig. 3 shows the cantilever bending and resonance frequency variation when a SAM-coated cantilever is exposed to PETN. As seen from Fig. 3, the bending response of the cantilever to the PETN exposure is extremely sensitive and fast. Because the noise level of the bending response in these experiments is 2 nm (3 standard deviation of the noise level), the detection sensitivity corresponding to Fig. 3 is 14 ppt. Maximum bending of the cantilever is achieved within 20 s.
Fig. 3. Response of a 4-MBA-coated silicon cantilever to PETN vapors of 1.4-ppb concentration in ambient air. The solid curve depicts the bending response, and the dots depict the resonance frequency of the cantilever. The frequency shift due to the adsorption of PETN vapor corresponds to a mass loading of 15 pg on the cantilever.
Fig. 2 shows the rapidity with which explosive vapors can be detected and the relatively fast relaxation of the cantilever when the vapor stream is turned off. When the PETN stream is turned on for 10 s, a 40-nm deflection signal is observed. When the vapor stream is turned off, the cantilever is relaxed, returning almost to its original position within 60 s. As mentioned previously, another important observation from the data shown in Fig. 2 is that the resonance frequency of the cantilever does not change significantly as a result of the small amount of PETN deposited in 10 s. The cantilever bending is still easily detected. It is assumed that the hydrogen bonding between the nitro groups of the explosives molecules and the hydroxyl group of 4-MBA is responsible for the easily reversible adsorption of explosive vapors on the SAM-coated top surface of the cantilever [30]. In the case of “active detection” of explosive vapors, the explosive vapor was allowed to deposit on a piezoresistive microcantilever, and a 10-V, 10-ms voltage pulse was applied to it. This led to the deflagration of the explosive material, which resulted in an exothermic “bump” on the microcantilever bending signal (the bending of the cantilever was monitored using optical detection) [53]. In addition, the smoke plume generated can be clearly seen for deposited TNT mass above a few hundred picograms. Fig. 4 shows a series of images captured with a high-speed camera [53]. Fig. 4(a) shows an image of a TNT-loaded cantilever before application of a voltage pulse. Fig. 4(b)–(d) shows a sequence of frames just after the heating pulse. Fig. 4(b)–(d) clearly shows the evolution of a plume of gas from reactions occurring on the cantilever; the smoke plume in the path of the laser light (used to monitor the cantilever bending) is illuminated, clearly displaying the smoke ring. Laser scattering off of the cantilever is hardly visible in Fig. 4(d), whereas it is clearly visible in Fig. 4(a). This can be explained as follows: Just before the application of the voltage pulse [Fig. 4(a)], the cantilever surfaces are covered with TNT,
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Fig. 5. Detection of a biowarfare agent. Cantilever bending response as a function of exposure time to ricin.
Fig. 4. High-speed photos of a TNT-deposited cantilever subjected to a voltage pulse leading to deflagration (a) just before the voltage pulse and (b)–(d) a sequence of frames just after the voltage pulse. The left (red) portion of the circular plumes in (b)–(d) are illuminated by the laser light used to monitor the cantilever bending. The amount of TNT deposited on the microcantilever was 1 ng (calculated from the observed shift in microcantilever resonance frequency).
which leads to higher specular reflection of the laser beam compared with that for a TNT-free cantilever in Fig. 4(d). The described “active detection” method for explosives, although not suitable as a component for a miniature sensor with multiple-threat detection capabilities, can be used in an explosive-detection sensor package to provide confirmation of a positive signal generated by the “passive detection” sensor array method. B. Biowarfare Agents Although not as prevalent as explosives, the use of chemical/biological agents as a warfare or terrorist weapon is a serious threat. Several biological agents exist that can be used as warfare agents. Examples of biowarfare agents include botulinum toxin, Shiga toxin, diphtheria toxin, anthrax, and ricin. Most biological agents are derived from bacterium. Ricin toxin is produced from
caster bean extract. Botulinum toxins are some of the most deadly substances known, These toxins are 100 000 times more toxic than the nerve agent sarin, 10 000 times more toxic than VX, and 1000 times more toxic than ricin [55]. The estimated lethal dose of a botulinum toxin (type A) is 1 ng/kg of body weight, and the lethal blood level of the toxin is around 20 pg/mL. The estimated lethal dose of ricin is 3 g/kg of body weight. At present, no widely available rapid test exists for most biological agents. We have successfully detected the biowarfare agent ricin using modified microcantilevers [56]. One side of the microcantilever was modified with ricin antibody. When ricin was introduced into a liquid cell housing the cantilever, the cantilever bent due to ricin-antibody interaction. Fig. 5 shows the cantilever bending response as a function of time after the introduction of ricin. The experiments were not done under flow conditions. The large response time is due to diffusion of ricin toward the cantilever. A key requirement for the detection of biological species is the ability to modify the microcantilever surface for biospecific recognition. However, most molecular-recognition-agent-containing molecules are not commercially available, and thus tremendous amounts of synthetic work must be done to develop each molecular-specific microcantilever surface. In this regard, a general microcantilever surface-modification method through layer-by-layer technology for biomolecule recognition was reported [57]. Weeks et al. [58] reported on the detection of specific Salmonella enterica strains using a functionalized microcantilever. C. Chemical Warfare Agents Basically, there are three types of chemical warfare agents: nerve agents, blister agents, and choke agents. Nerve agents affect the transmission of nerve impulses in the nervous system. All nerve agents belong chemically to the group of organophosphorus compounds. Examples of nerve agents are GA (tabun), GB (sarin), and GD (soman). Blister agents burn and blister the skin or any other part of the body they contact. They act on the eyes, mucous membranes, lungs, skin, and blood-forming organs. They damage the respiratory tract when inhaled and cause vomiting and diarrhea when ingested. The blister agents include sulphur mustard (HD), nitrogen mustard (HN), and Lewisite (L). Choking agents are chemical agents that attack
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Fig. 6. (Left) Schematic representation of the bending of a self-assembled Cu =L-cysteine bilayer-coated microcantilever upon complexation with DMMP. (Middle) Bending response as a function of time, t, for a cantilever coated with a self-assembled Cu =L-cysteine bilayer on the microcantilever after injection of 10 M solutions of various electrolytes. (Right) Maximum deflection of a silicon cantilever coated with a self-assembled Cu =L-cysteine bilayer on the gold surface as a function of the concentration of DMMP in 0.01-M tris buffer at pH = 5:0.
lung tissue, primarily causing pulmonary edema. In low concentrations, choking agents act on the respiratory system to cause an accumulation of fluid in the lungs, which can lead to death. In high concentrations, choking agents lead to death for the same reason, but they might also affect the upper respiratory tract. Chemicals classified as choking agents are chloropicrin (PS), chlorine (Cl), phosgene (CG), and diphosgene (DP). Chemical warfare agents have been used in the past. Nerve agents are among the most toxic of known substances. The nerve agent—either as a gas, an aerosol, or a liquid—enters the body through inhalation or through the skin. Poisoning may also occur if liquids or foods contaminated with nerve agents are consumed. If a person is exposed to a high concentration of a nerve agent (e.g., 200-mg sarin/m ), when the agent is absorbed through the respiratory system, death may occur within a few minutes. When the nerve agent enters the body through the skin or through consumption, death is less sudden. L-cysteine on a gold A self-assembled bilayer of Cu surface has recently been characterized [32] and could be used to recognize phosphonyl groups because of the formation of L-cysteine strong P O Cu bonds. We have used a Cu bilayer-modified cantilever to detect nerve agents in aqueous solution based on this mechanism. Dimethyl methyl phosphonate (DMMP) was used as a sarin nerve gas simulant [32]. L-cysteine bilayer was formed by The coating of the Cu M solution of L-cysteine immersing the cantilever into a ) for 24 h. The microcantilever in tris buffer solution (pH was then rinsed with tris buffer solution and immersed in M CuSO tris buffer solution for another 24 h. When DMMP was introduced into the cantilever chamber, the cantilever underwent bending due to interaction. When the DMMP was replaced with tris buffer solution, the cantilever did not return to its original position. The flow rate was 4 mL/h. Fig. 6 shows the cantilever bending response as a function of DMMP concentration [32].
It has been shown that phosphonyl groups strongly bind with Cu and copper complexes. Organophosphorus compounds are unstable at high pH levels. The cantilever bending caused by exposure of DMMP is most likely the result of complexaL-cysteine bilayer on the microtion of DMMP with the Cu cantilever surface through Cu O P bonds that alter the surface stress. A cantilever deflection of almost 5 nm can be M. The obdetected even for a DMMP concentration of served intereference from analytes such as sodium phosphate, DL-aspartic acid, dimethylamine, 1,10-phenanthroline, acetic acid, and acetonitrile was negligible. D. Nuclear Radiation So far no release of radioactive materials has occurred due to terrorism. However, the threat of dirty bombs made of explosives and radioactive materials is still a possibility. The possibility of nuclear materials on the black market from other countries increases the threat level and emphasizes the need for improved radiological sensing devices that can be massively deployed. Dirty bombs could be detected with an explosive-vapor detector. We have also successfully demonstrated a micromechanical radiation detector for alpha particles [59]. The detection was carried out in air. In this experiment, alpha particles were allowed to impinge on an electrically insulated metallic surface of 1-mm diameter. A microcantilever that is kept at a fixed distance of a few nanometers undergoes deflection as a function of residual charge accumulation on the surface. The static deflection method suffers from interference from thermal drift. To avoid thermal drift, we have used the resonance response of a cantilever. Transients produced by continuously running force calibration curves on the AFM microscope accumulated the resonance transient. In an AFM, force calibration is achieved by pushing the sample (collector sphere in this case) against the cantilever and then withdrawing it. As it is withdrawn, the
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cantilever tip does not initially break free of the surface because of tip-surface adhesive forces; instead, the cantilever bends until its restoring force (due to its spring constant) exceeds the adhesion force. Because tip-surface attraction decreases more rapidly than the restoring force with increasing tip-surface distance, the cantilever rapidly accelerates away and enters the harmonic resonance mode predicted by its mass and spring constant. This transient resonance signal gradually loses its energy (e.g., to air or through internal friction) and comes to rest. Transient oscillations were recorded on a digitizing oscilHz using an output signal from the deflection loscope at measurement system and then analyzed using the appropriate software. The force-distance curve serves as a measure of distance between the cantilever and the surface. Because the force gradient plays a major role in the cantilever resonance frequency, precise knowledge of the distance between the tip and the surface is essential for avoiding error in frequency-response measurements. We have detected alpha particles using the shift in resonance frequency of the microcantilever due to electrostatic force (nonuniform fields produce a change in resonance frequency due to modification of the force constant of the cantilever as the result of field gradients). Using this method, a single alpha particle can be detected, provided the frequency is measured with Hz. Frequency, however, can be detected a sensitivity of with even higher sensitivity using available electronics. The device can be optimized using large area collectors. We have also used bending variation as well as variation in damping of cantilever frequency to detect alpha particles. These techniques have sensitivities of the same order of magnitude as described above. Fig. 7 shows the resonance-frequency variation (measured from transients) as a function of exposure to alpha particles [59]. Until now, most studies on microcantilever sensors have been conducted to illustrate that specific microcantilever coatings can be developed to detect various species with very high sensitivity. However, to develop a miniature sensor system that can simultaneously detect a wide variety of threats without operator assistance, the following basic capabilities are required. • To selectively detect a variety of threats (or even a variety of chemical species), a cantilever array consisting of tens or hundreds of cantilevers may be needed. • The signal transduction method must be simple and compact requiring no operator assistance. It is not sufficient to have a MEMS sensor element if the signal transduction cannot be miniaturized. • Telemetry must be built into the design so that after a threat is identified, it can be transmitted instantaneously to a monitoring station. We discuss the status of these capabilities in the following sections.
V. CANTILEVER ARRAYS Compared with other sensor technologies, relatively few studies have been conducted with microcantilever arrays [26], [60]–[67]. Out of these, pattern recognition algorithms were
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Fig. 7. Nuclear radiation detection. Resonance frequency variation of a cantilever as a function of exposure to alpha particles.
used to identify components of vapor mixtures in only two studies [62], [63], in which simple mixtures were analyzed using an eight-cantilever array; it is interesting to note that the microcantilever bending signal was monitored in both studies, using optical detection. Therefore, the feasibility of identification of components in a simple mixture has been illustrated [62], [63] with the microcantilever platform. Based on the lessons learned from the chemical sensor array studies conducted up to now (for example, [2], [6], [9], [11], [34]), some or all of the following may be needed to improve the detection capabilities of a microcantilever sensor array so that it can detect trace amounts of multiple analytes: vapor preconcentration, careful control of experimental conditions to provide a “good memory” for pattern recognition, and inclusion of a wide variety of coatings to provide a sufficient signal variation. Detection speed is challenging, especially for chemical, biological, and radiological exposures. For instance, a few seconds may be all the time available to respond to threat-level quantities of a nerve agent. Especially with the slower reaction rates of biological agents, detection times of seconds to minutes could limit the amount inhaled and simplify subsequent prophylactic action. Even though the enhanced sensitivity of microcantilevers is due primarily to their small size (i.e., the large surface-to-volume ratio that greatly amplifies the bending signal), the small size also decreases the probability of target molecules being captured onto the sensor surface. This loss could be compensated for by using a preconcentration system at the front end of the microcantilever sensor. Preconcentrators that can rapidly bring a sufficient quantity of agent into the detection volume of the cantilever element could be essential for significantly decreasing the detection time. VI. PIEZORESISTIVE DETECTION OF MICROCANTILEVER BENDING SIGNAL—A SIGNAL TRANSDUCTION METHOD SUITABLE FOR A MINIATURE SENSOR When making a miniature sensor, it is not enough just to have a small sensor element. Signal transduction and transmission capabilities also need to be incorporated into a small package. In this section, we briefly discuss a suitable signal transduction method for microcantilevers.
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Since the advent of the AFM, several signal transduction methods have been explored for monitoring microcantilever deflections. These methods include optical, piezoresistive, piezoelectric, and capacitive methods [68]. In the optical method, a laser diode is focused at the free end of a cantilever. The reflected light is detected with a position-sensitive detector (PSD). In the piezoresistive method, the silicon cantilever is doped with boron or phosphorous. The electrical resistance of the doped channel changes as a function of cantilever bending. In the piezoelectric method, cantilever bending causes a transient charge on a piezoelectric film, such as ZnO, on the cantilever. Because the signal is transient, it is not ideal for static cantilever-bending measurements. In the capacitive method, the capacitance between the cantilever, which is micromachined with a space between the cantilever and the substrate, is measured. The piezoresistive microcantilever readout method was originally developed by Tortonese et al. [69] for atomic force microscopy. Subsequently, Boisen et al. [70], [71] developed a microcantilever chip with four piezoresistive microcantilevers specifically for sensor applications. Such a microcantilever chip has recently been commercialized by Cantion, Inc., Denmark. This piezoresistive microcantilever platform is ideal for the proposed single-sensor platform for the reasons discussed below. We recently developed [32] a handheld sensor that makes use of the four-microcantilever (Canti-4) chips from Cantion. Two cantilevers in the Canti-4 chips are coated with gold (a 30-nm-thick gold layer on top of a 3-nm chromium adhesion layer). In these experiments, we used only the two gold-coated microcantilevers, one of which was coated with a 4-mercaptobenzoic acid SAM [29]. We have named our first handheld sensor “SniffEx” because it is being developed for the detection of explosive vapors. A photograph of SniffEx is shown in Fig. 8. The differential signal between the two gold-coated microcantilevers was obtained by using those two microcantilevers in a Wheatstone bridge circuit [31]. The Canti-4 chip is located in an aluminum flow cell observed at the top of Fig. 2. A 1/16-in. stainless steel tube is connected to the input of the flow cell. In “sniffing” experiments, a small pump (shown at the top left of Fig. 8) connected to the exit port of the flow cell is used to extract vapor from a sample bottle [31]. In experiments in which a calibrated explosive-vapor generator developed at INEEL [31] was used, the pump was turned off and the inlet tube was connected to the INEEL vapor generator, which delivered low concentrations of explosive vapor at a flow rate of 50 standard cubic centimeters per minute (sccm). Fig. 9 shows the response of the SniffEx to a calibrated RDX vapor stream in nitrogen carrier gas at a flow rate of 50 sccm. Three sets of data taken on three separate days with the same cantilever chip are shown for a RDX vapor concentration of 6 ppt. These show that data obtained under such controlled conditions have good reproducibility. The noise level (standard deV, viation of the background) for the data of Fig. 9 is and thus, the detection limit for a single measurement is 1.8 V (3 standard deviation). This limit corresponds to RDX vapor concentrations well below the ppt level (less than 1 part in ). It is clear that the piezoresistive microcantilever sensor has extremely high sensitivity.
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Fig. 8. Photograph of the SniffEx handheld sensor [31]. The flow cell is located at the top of the figure; the small pump used to extract vapor is shown on the top left; the battery powering the pump is on the top right. The battery to power the electronics is located at the bottom. Although normally the power comes from the computer’s serial port, this battery will be used when the computer is replaced by a personal data assistant (PDA). Communication with a laptop computer is via the serial port located at the bottom of the handheld unit.
Fig. 9. Response of SniffEx to a calibrated RDX vapor stream from the INEEL vapor generator [31]. The carrier gas was nitrogen at 50-sccm flow, and the generator temperature was 25 C, which corresponds to an RDX concentration of 6 ppt. Three data sets obtained on three separate days are shown. A signal strength of 1 mV corresponds to a differential surface stress of 1.25 N/m.
As shown in our measurements on the detection of the vapors of the plastic explosives, the detection sensitivity for the piezoresistve transduction method [31] is comparable with that for the optical transduction method [30], which is considered
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to be the most sensitive signal transduction method for microcantilevers. The use of a reference cantilever for each coated cantilever, which is inherent in the Wheatstone bridge measurement used with the SniffEx [31], provides common-mode rejection and eliminates sensor drift caused by external effects such as temperature and pressure variations. It is possible to mass fabricate sensor chips with hundreds of pairs of such microcantilevers on a single chip. These features, together with the capability to incorporate the all-electrical detection circuit on a single chip, makes the piezoresistive transduction method the best suited for an easy-to-use miniature sensor. VII. TELEMETRY Telemetry has also made significant advances in the past decade. Many digital instruments, including wireless monitoring instruments, are in use today. But the sensors do not take full advantage of silicon technology partly because they are analog in nature. The development of an application-specific integrated circuit (ASIC) with built-in telemetry (i.e., a telesensor) may not be available in the near future [72]. However, a miniature sensor package with telemetry that is not on a single chip can currently be built. Therefore, it is feasible that in a few years a cantilever-based, integrated miniature sensor (CIMS) could be used for limited applications in explosive-vapor detection and some chemical vapor detection. It could take longer to extend the use of CIMS to detect a broad spectrum of chemical, biological, radiological, or explosive (CBRE) threats because of the complexities involved. As the capability to incorporate more detection capabilities grows, the cost of CIMS will decrease because of increased deployment, analogous to the technology of silicon-integrated circuits that has progressed according to Moore’s Law. The fabrication of a true telesensor chip with built-in telemetry will get us there much faster. VIII. CONCLUSION In the war on terrorism, the capability to detect CBRE weapons in advance and the capability for the early detection of chemical or biological agents from an attack that already occurred should be at the forefront of a long-term strategy. This requires omnipresent miniature sensors that can detect multiple terrorist threats with high sensitivity and selectivity and that can relay the warnings instantaneously. Studies conducted on microcantilever sensors, together with the technological advances in neural analysis and telemetry, indicate the feasibility of a single-sensor platform based on silicon microcantilevers. Their simplicity of operation, high sensitivity of detection, miniature size, and flexibility to detect a wide variety of terrorist threats, together with low cost of production, make the microcantilever-based sensors an attractive solution. ACKNOWLEDGMENT The authors would like to thank Dr. X. Yan, Dr. V. Boiadjiev, Dr. F. Tian, Dr. G. Muralidharan, Dr. A. Wig, Dr. P. Oden, Dr. M. Doktycz, Dr. R. Warmack, Dr. C. Britton, Dr. D. Hedden, Dr. J.
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PINNADUWAGE et al.: MOORE’S LAW IN HOMELAND DEFENSE
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Lal A. Pinnaduwage (M’97) received the Ph.D. degree in atomic physics from the University of Pittsburgh, Pittsburgh, PA, in 1986. He is a Senior Staff Scientist of the Nanoscale Science and Devices Group, Oak Ridge National Laboratory, Oak Ridge, TN. He is also a Research Professor with the Department of Physics, University of Tennessee, Knoxville. He research interests include interaction of laser and microwave fields with fast atom beams, Rydberg atoms and molecules, negative ions, electron-excited molecule interactions, gaseous dielectrics, low-temperature plasmas, and mass spectrometry. He is currently working in the areas of MEMS sensors for explosive and chemical detection and surface physics. Dr. Pinnaduwage is a Fellow of the American Physical Society.
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Hai-Feng Ji received the Ph.D. degree from the Chinese Academy of Sciences, Beijing, China, in 1996. After one year of postdoctoral study with the Department of Chemistry, University of Florida, Gainesville, he joined Oak Ridge National Laboratory, Oak Ridge, TN, for a two-and-half-year postdoctoral stay. He has been an Assistant Professor of chemistry since 2000 with the Department of Chemistry and the Institute for Micromanufacturing, Louisiana Tech University, Ruston. Since 1995, he has published over 50 scientific papers in peer-reviewed journals with a focus on optical and micromechanical sensors. His main research interests include chem/bio-microelectromechanical systems (MEMs) and nanoelectromechanical system (NEMs).
Thomas Thundat received the Ph.D. degree in physics from the State University of New York at Albany in 1987. He is a Distinguished Staff Scientist and Leader of the Nanoscale Science and Devices Group, Oak Ridge National Laboratory, Oak Ridge, TN. He is also a Research Professor of physics at the University of Tennessee, Knoxville, and a Visiting Professor at the University of Burgundy, France. He is the author of over 170 publications in refereed journals, 18 book chapters, 18 patents, and nine patents pending. His research interests include novel physical, chemical, and biological detection using micro- and nanomechanical sensors. His expertise includes physics and chemistry of interfaces, solid-liquid interfaces, biophysics, scanning probes, nanoscale phenomena, and quantum confined atoms. Dr. Thundat is the recipient of many awards, including the U.S. Department of Energy’s Young Scientist Award, R&D 100 Award, Discover Magazine Award, FLC Award, ASME Pioneer Award, and UT-Battelle Awards for invention, publication, and research and development.
